Fuel cell power plants include one or more stacks of electrochemical cells which are operable to convert certain reactants to DC electrical power. The more common types of fuel cell power plants such as acid electrolyte cells utilize hydrogen derived from a hydrocarbon fuel such as natural gas, and oxygen typically derived from air, as the active ingredients in producing the electrical power. The reaction which occurs in each cell in a fuel cell power plant produces electric power, water and heat. In order to perform optimally, the fuel cells in the power plant should be operated at an optimal temperature which is nearly uniform across each cell in the stack. This temperature can theoretically be achieved by selecting the proper coolant and using the selected coolant to maintain the proper cell operating temperature.
Present day, larger capacity fuel cell stack assemblies use water-based coolants to control cell operating temperatures. The coolant must be able to prevent the cells from overheating, and at the same time, allow them to operate at high enough temperatures needed to make efficient use of the reactant supply. The ideal situation is to use a water-base coolant which will be converted to steam, or a mixture of steam and water at the optimum operating temperature of the stack cells. Achieving such ideal results can, however, increase power plant cost due to the need to include excessive or complex heat exchangers in the coolant loop to properly control the temperature of the coolant. The coolant composition can be varied so as to vary their boiling points; for example, cells operating at higher temperatures can be cooled with a mixture of water and a higher temperature boiling point liquid, such as glycol, or the like, in appropriate proportions. Likewise, cells operating at lower temperatures can be cooled with water that is essentially devoid of the higher temperature boiling point components. Regardless of what the exact composition of the coolant is, it is still desirable to bring the coolant to its boiling point as soon as possible after it enters the active area of the stack.
A problem which can occur in certain fuel cell power plants, which may depend on the ambient temperature at which they operate, is that when one achieves the aforesaid goal of boiling the coolant as soon as possible after it enters the actual area of the stack, the result may be to produce an excessive amount of steam in the coolant stream as it leaves the actual area of the stack. Since the coolant loop of a fuel cell power plant is essentially a closed loop, any steam produced will have to be condensed back to a liquid state, used in the steam reformer portion of the power plant, and vented into the ambient surroundings. Once again, the production of excess amounts of steam in order to accomplish the uniform cell temperature objective will add to cost of the power plant due to the need to recondense or purge the steam thus created. It will be appreciated from the aforesaid that the only steam that is produced during cell cooling that can be of direct benefit to the operation of the fuel cell stack is that used to reform the natural gas stream. In certain cases it would, therefore, be highly desirable to have a cell stack cooling system which would ensure quick boiling of the coolant as it enters the active area of the stack, while at the same time, would ensure the production of an amount of steam in the coolant leaving the active area of the stack necessary to operate the power plant steam reformer.